1. Field of the Invention
The invention is related generally to the field of drilling wellbores through earth formations. More specifically, the invention is related to devices used to plug back the lower part of a wellbore.
2. Description of the Related Art
It is often desirable, for various reasons, to partially abandon a wellbore which is drilled through certain earth formations. Sometimes partial abandonment is for the purpose of redrill the wellbore through the earth formations along a different path or trajectory. For example, the wellbore operator may wish to test the earth formations at a different position on a geologic structure. In other cases drilling apparatus may have become irretrievably stuck in the lower portion of the wellbore and must be abandoned. When the wellbore operator desires to use a part of the wellbore as originally drilled, rather than abandoning the wellbore entirely, the operator can seal off or "plug" the portion of the wellbore he wishes to abandon, and then can use the unplugged part of the wellbore above the plugged portion to act as a conduit for drilling a new part of the wellbore along a different trajectory. The process of drilling the new part of the wellbore along a different trajectory is known in the art as drilling a "sidetrack", or "sidetracking".
Sidetracking can be performed in a number of different ways, depending on, among other things, whether the remaining (non-abandoned) portion of the wellbore has steel pipe ("casing") installed therein. Methods known in the art for sidetracking include setting a device known as a "whipstock" at the depth in the wellbore where the operator desires to start the sidetrack ("kick-off point"). See U.S. Pat. No. 5,222,554 issued to Blount et al for one example of a whipstock.
Whipstocks are frequently used to start the sidetrack where the wellbore has casing installed. Certain types of whipstocks can be "set" or installed in a cased wellbore in a desired orientation by some form of frictional contact with the casing to prevent rotation. The Blount et al '554 patent shows a typical whipstock having a "setting" mechanism. It is necessary to orient the whipstock rotationally so that the sidetrack will start in the desired azimuthal direction.
Other examples of a whipstock are shown, for example, in U.S. Pat. No. 4,182,423 issued to Ziebarth et al, for use in cases where the wellbore does not have casing. If the sidetrack is initiated from a part of the wellbore which does not include casing (called "open hole"), it is usually desirable to plug the abandoned part of the wellbore by filling it with cement or the like. The whipstock can then be set on the top of the cement after curing, and the sidetrack can be initiated. The whipstock shown in the Ziebarth et al '423 patent includes an adhesive backing and a deformable element to push the adhesive side of the whipstock into contact with the wellbore wall so as to eliminate the need for plugging the open hole.
More recently, it has become common in wellbore drilling to simply fill the abandoned portion of the wellbore with cement and start ("kick off") the sidetrack using directional drilling tools known as "steerable" or "bent housing" motors. These are hydraulically powered drilling motors which have a housing typically subtending an angle of 1/2degree to 2 degrees along the housing's length (generally about 30 to 40 feet). Such motors are well known in the art for drilling wellbores along a desired trajectory, and to a great extent have replaced whipstocks as a mechanism for starting a sidetrack and drilling a well along a selected trajectory. The steerable or bent housing motor saves the operator time by eliminating an extra "trip" in the wellbore to set and orient the whipstock.
Typically when kicking off a sidetrack using a steerable motor or bent housing motor, the abandoned part of the wellbore is simply filled with cement and the operator waits for the cement to harden before beginning drilling with the steerable motor. In some cases the earth formations at the kick off point are substantially harder than the cement, or the cement may not set to a sufficient compressive strength or may not have uniform hardness over the entire volume of the cement plug. In such cases it may be difficult to start the sidetrack using a bent housing or steerable motor. Soft, or non-uniform cement may be preferentially drilled by the drill bit, and the motor may not be able to "steer" the bit properly to start a sidetrack.
Precast or preformed plugs to start sidetracks are known in the art. U.S. Pat. No. 2,281,414 issued to Clark shows an example of a combined bridge plug and drillable whipstock. U.S. Pat. No. 2,509,144 issued to Grable et al shows a well plugging and whipstock device. U.S. Pat. No. 2,119,746 issued to Lane shows a precast plug-type whipstock used to reestablish verticality of a wellbore which has begun to deviate from vertical. Each of these prior art plugs may require cement to be pumped into the wellbore in a separate step in order to provide sufficient strength to the plug to enable starting a sidetrack, particularly when using steerable or bent housing motors. The additional cementing step can be time consuming, costly, and sometimes ineffective for bonding a precast plug to the wellbore.